We propose to investigate the biosynthesis of ACTH and the secretion of ACTH by the corticotrophs of the adenohypophysis. Biosynthesis and secretion will be examined in suspensions of isolated adenohypophysial cells. The corticotrophs in these isolated cells respond to CRF(s) with increased secretion of ACTH into the incubation medium. The system is an accurate, highly sensitive and apparently specific method for the assay of CRF(s). CRF-induced secretion is inhibited by glucocorticoids. The system offers a means of elucidating the mechanism of inhibition by the glucocorticoids and the process involved when the responsiveness of the corticotrophs to CRF increases in the absence of steroid. The mechanisms and processes involve steroid repression and depression of RNA synthesis and of changes in protein synthesis. The results of the study have important implications to the understanding of the factors which determine the fluctuations in the rate of release of ACTH from the pituitary of man under normal and in disease states. ACTH equals Adrenocorticotropic hormone. CRF equals Cortocotropin releasing factor.